Small-scale BGP customer prefix filtering
At this small scale, manually configuring a
prefix-list for your BGP customer is appropriate.
What larger transit networks do
Larger transit networks consume IRR database(s) (RADB FAQ link) to automatically generate such prefix-lists. However, for just one BGP customer having one prefix-list entry, that kind of automation investment would be unwise.
What your upstream transit providers do, and how that affects you now
Note that you may need to think about this from your transit providers' perspective as well -- how are they managing your prefix-list on their equipment; and how does your new BGP customer's entry get into that list?
Do you manually request they update the list; or do you publish the appropriate IRR entries, and they consume them?
If your transit providers require you maintain IRR entries
If the later, you'll want to add your new BGP customer to an IRR
as-set: object containing both your ASN and theirs; and ensure appropriate
route: objects exist matching each origin ASN.
If they require it and do it on your behalf
If your transit providers simply do this on your behalf, it will create an IRR database maintenance headache for you in the future. It's best to control your own IRR objects; not have to email former ISPs to get them updated / deleted later on.